In contrast to the relatively quiet fissure eruptions of flood
basalts, at various times more violent eruptions of rhyolitic lava hurled vast
quantities of ash and other volcanic material into the air. The resulting rain
of volcanic debris covered extensive areas and formed deposits that because of
their fragmental character, superficially resemble sedimentary deposits. Rocks
resulting from the consolidation of such volcanic deposits are known as
pyroclastic rocks; those formed from very fine material, such as ash, are called
tuffs. Pyroclastic rocks are not abundant on Isle Royale but do occur sandwiched
between some of the flood basalt flows.

THE WAY THE ROCKS ARE STACKED UP

The bedrock sequence on Isle Royale consists of a thick pile of lava flows and
sedimentary rocks that has been tilted toward the southeast. Having already
looked briefly at the different varieties of rocks that occur in the pile, we
can now examine the rock sequence in more detail and get to know the subtleties
of its character and the relative position or succession of its component parts
- its stratigraphy.

In geology, a formation is an assemblage of rocks that have some character in common, whether of origin or composition. It is thus a way of dividing rocks
into manageable units for discussion or for depiction on a geologic map. The
rock sequence on Isle Royale has been divided into two formations, one deposited
above the other. The lower - and thus older - one has been named the Portage Lake
Volcanics and includes all the lava flows and their minor interbedded
sedimentary and pyroclastic rocks. The upper formation, the Copper Harbor
Conglomerate, contains only sedimentary rocks; it is mostly conglomerate but
also includes sandstone. On the Keweenaw Peninsula of Michigan, where these
formations also occur and were originally named, other formations can be seen
both above and below the Portage Lake Volcanics and Copper Harbor Conglomerate.
Some of the other formations undoubtedly also extend to the vicinity of Isle
Royale, but they are concealed beneath the waters of Lake Superior.

A VOLCANIC PILE --- THE PORTAGE LAKE VOLCANICS

There are probably over 100 individual lava flows in the Portage Lake Volcanics
on Isle Royale with a total thickness of over 10,000 feet - quite an impressive
pile. And the total thickness of the formation must be greater because although
the top of the formation is exposed, the bottom is not, and an unknown number of
additional flows undoubtedly lie beneath Lake Superior on the north side of the
island.

The sedimentary rocks interbedded with the lava flows are exposed only under
unusual circumstances because they are more easily eroded than the volcanic
rocks and as a result are generally buried beneath surficial materials in depressions between ridges of more
resistant volcanic rock. We know from holes drilled during exploration for
copper deposits that about 25 sedimentary layers 1 foot thick or more are
present in the sequence; however, less than a third of these layers are known
from outcrop, and all those are in the upper part of the sequence.

In order to construct a geologic map, which shows the geographic distribution of
individual rock units and their relationships to each other, some method must be
found of distinguishing key rock units from each other; usually, rock
composition does the trick. But within a pile of similar lava flows, this is a
distinct problem. Fortunately, some of the lava flows in the Portage Lake
Volcanics can be distinguished from their neighbors above and below on the basis
of their distinctive rock textures. These flows can then be used as divisions or
marker units within the sequence of flows and can provide stratigraphic and
structural control for depicting the sequence on a geologic map. The interbedded
sedimentary rocks would also serve as marker units for mapping if they were
exposed more frequently.

The amygdaloidal zones at the base and top of flows are less resistant than the
massive flow interiors; therefore, the interior parts of the thicker flows are
the dominant ridge-forming units. Contacts between flows, on the other hand, are
seldom exposed as they are generally concealed beneath alluvial materials or
talus in the depressions between ridges. Fortunately for geologic mapping, the
distinguishing texture of an individual flow is generally better developed and
more readily apparent in the nonamygdaloidal flow interiors, and therefore, that
part of a flow most useful for purposes of identification is invariably the most likely part to be exposed.

The volcanic rocks on Isle Royale are predominantly ophitic, and except for a
few unusual ophites, the nonophitic flows are most conspicuous among their
neighbors and make the best horizon markers. The finer grained porphyrites and
traps are also generally more resistant to erosion than ophitic flows of similar
thickness and thus tend to stand out topographically, making it easier to trace
them across the countryside and adding to their usefulness in geologic mapping.
There are 12 flows within the Portage Lake Volcanics on Isle Royale that are
distinctive enough to be easily separated from the rest of the flows in the
pile, and these have been named for ease of reference (fig. 17). Most of these
named flows are less than 100 feet thick, but six of them can be traced the
length of the island. The named flows include five porphyrites, four traps, and
three ophites. Only four of these individual flows are shown on the geologic map
in this volume (pl. 2), but all are shown on the larger scale geologic map
published separately (Huber, 1973c).

For most visitors to Isle Royale, the northeast and southwest ends of the island
are the most accessible, although a growing number of visitors are hiking the
length of the island and the Greenstone Ridge Trail, which follows Greenstone
Ridge, the backbone of the island. At neither end of the island can all the
named lava flows or volcanic rock types be readily seen, but enough variety is
present to provide interesting geological excursions from Rock Harbor Lodge and
Windigo Inn via small boats available at both places. A somewhat less varied
assortment of rocks can be seen on short hikes from the lodge areas.

Most of the rock types in the Portage Lake Volcanics can be seen in the Rock
Harbor - Tobin Harbor area (pl. 1). The main exception is the coarse porphyrite
characteristic of the Huginnin Flow (fig. 12D), which can only be seen on the
north side of the island.

All the volcanic rocks on the chain of islands on the south side of Rock
Harbor are ophitic, and excellent exposures can be found on the wave-buffeted
south shores of those islands. Ophitic texture is especially well developed on
the south side of Raspberry Island where exposures are readily accessible via
the Raspberry Island Nature Trail (fig. 13 is from this locality). Ophite can
also be seen along the Commodore Kneutson Trail near the Rock Harbor Lodge.

Fine-grained porphyrite can be seen in the Scoville Point Flow at various points
on the north shoreline of Rock Harbor, as at Threemile Campground, on Scoville
Point, and on the south sides of Edwards and North Government Islands. Its
speckled appearance is quite distinctive (fig. 12B). Rock with a somewhat
similar texture (fig. 12C) can be seen in the Tobin Harbor Flow at various
points along the north shoreline of Tobin Harbor, as near the dock for the
Lookout Louise Trail, on several small islands in Tobin Harbor, and on the south
side of Porter Island. The trail to Mount Franklin also surmounts a bold outcrop
of the Tobin Harbor Flow shortly after crossing Tobin Creek.

Fine-grained trap of the Edwards Island Flow can best be seen on the small
promontory just north of Scoville Point, on Split Island, and on the north sides of Edwards and North Government Islands. Trap of the Long Island Flow can
be seen on several small islands opposite the dock for the Lookout Louise Trail
and on Long and Third Islands. The Edwards Island Flow and to a lesser extent
the Long Island Flow have an important secondary attribute that helps in their
field recognition; except for the upper part of the Greenstone Flow, they are
the only flows in the volcanic sequence that commonly exhibit well-developed
columnar jointing (fig. 18).

COLUMNAR JOINTING IN EDWARDS ISLAND FLOW, Edwards Island, (Fig. 18)

Columnar
joints are cracks that divide a lava flow into long vertical columns ideally
tending toward a hexagonal cross section. They are formed during cooling of the
flow under certain specific conditions. Rapid and uniform cooling rates tend to
promote the development of columnar joints, but the degree of homogeneity of the
solidifying rock is probably more important. It can be shown mathematically that
the surface of a homogeneous medium should be divided by a crack system defining
regular hexagons when it is subjected to uniform tension because a hexagonal
system provides the greatest stress relief with the fewest cracks. Regular hexagons are rare, however, because cooling stresses in rocks
are never completely uniform and the columns are generally bounded by curved
cracks forming irregular-shaped polygons with variable numbers of sides.
"Ideal" cooling conditions are never reached at the surface of a flow.
However, as a progressive zone of cooling, solidification, and cracking proceeds
from the surface of a flow into its interior, a point may be reached where the
shrinkage forces may be uniform enough for earlier irregular jointing to give
way to the formation of columnar joints. The importance of the homogeneity of'
the cooling rock explains why in the Portage Lake Volcanics well-developed
columnar jointing is restricted to the very fine grained traps and has only a
very slight tendency to form in the coarser grained porphyritic and ophitic
rocks.

Columnar jointing can be seen in the Edwards Island Flow at all the localities
mentioned previously. Other accessible areas where columnar jointing can be
observed are where the trail from Rock Harbor to Mount Franklin crosses the flow
and on the south slope of Ransom Hill just west of the trail from Daisy Farm
Campground to Mount Ojibway.

The Greenstone Flow is the thickest flow on the island and holds up the most
prominent ridge running the length of the island, Greenstone Ridge. The
Greenstone Ridge Trail, which follows the ridge for nearly its full length,
provides access to numerous scattered outcrops, but only at the far northeast
end of the island near Blake Point is a reasonably complete cross section of the
flow exposed. There the flow can be seen to consist of four divisions with
approximate thicknesses as indicated: a lower ophitic zone (100 ft), a central
pegmatitic zone (75 ft), an upper ophitic zone (175 ft), and an uppermost columnar-jointed trap (50 ft), for a total
thickness of about 400 feet. The Greenstone Flow attains its greatest thickness
in the central part of Isle Royale, where it is estimated to be nearly 800 feet
thick.

The lower ophitic zone, which makes up the cliffs of the Palisades on the north
side of Blake Point, is of particular interest as it is matched in coarseness
only by the ophite of the Hill Point Flow; each has augite crystals as much as 2
centimetres in diameter. The pegmatite is best seen in the vicinity of the
lighthouse on Blake Point. The columnar jointed trap that forms the uppermost
part of the Greenstone Flow is exposed on the string of small islands south of
Merritt Lane, on Red Rock Point, and at a few additional isolated localities on
the main island.

An excursion to Blake Point is also a botanical experience, as a rather unusual
shrub fills a small ravine just west of the lighthouse - devilsclub. The entire
plant, including its giant leaves, is profusely armed with sharp spines, and
anyone who ventures to walk through a thick stand of these would appreciate the
plant's specific name -- Oplopanax horridum (fig. 19). It presence on Isle
Royale is truly remarkable, for it appears nowhere else in the United States
east of the Rocky Mountains; at Isle Royale it is found only near Blake Point,
on Passage Island, and on a few of the small islands in the Rock Harbor area.

Pyroclastic and sedimentary rocks are exposed in only a few localities in the
Rock Harbor - Tobin Harbor area. Pyroclastic rock can be seen on the north shore
of Tobin Harbor opposite Newman Island and on Porter Island above (southeast of)
the columnar jointed upper part of the Greenstone Flow. The rock is a tuff-breccia,
composed of angular fragments of volcanic rock cemented together by ash (fig.
20). The ash was in a hot and partly plastic state when deposited and was then
fused into a coherent mass upon cooling.

An extensive wave-swept outcrop of conglomerate is exposed at the southwest end
of Moth Island, and a few other less accessible outcrops are scattered along the rest of the chain of
islands south of Rock Harbor (fig. 21). Volcanic rocks of many varieties can be
identified as pebbles in the conglomerate. Many of them are of rhyolite or other
types different from those that form the bulk of the Portage Lake volcanic
sequence and must have been derived elsewhere.

LAKE SUPERIOR BASIN - cross section. (Fig. 38)

ISLE
ROYALE A SMALL PIECE OF THE BIG PUZZLE

The orientation of the beds on the Keweenaw Peninsula is a mirror image of' that
on Isle Royale. The bedrock sequence, similar to that on Isle Royale, generally
dips northwestward toward the axis of the Lake Superior basin. Recognition of
these similarities permitted C. T. Jackson to state as early as 1849 that
"this island has the same geological character as Keweenaw Point, and is of
the same geological age." He and his colleagues, J. W. Foster and J. D.
Whitney, furthermore considered the rock sequences in the two areas to be
connected beneath Lake Superior and thus interpreted the lake as occupying a
structural basin, a syncline, as well as a topographic depression (fig. 38).

The later work of A. C. Lane not only reinforced this interpretation but also
demonstrated that some individual lava flows or groups of flows, as well as some
sedimentary rock units, on Isle Royale were the same as those on the Keweenaw Peninsula (fig. 39). The
continuity of the Greenstone Flow across the Lake Superior basin is most
convincing, and its name is used in both localities; specific correlation of
other flows is less certain, and separate names are applied to them on opposite
sides of the lake.

The Portage Lake Volcanics on Isle Royale and on the Keweenaw Peninsula
represent only the upper part of the total Keweenawan volcanic sequence in the
Lake Superior region. The lower part of the sequence is exposed in the so-called
South Trap Range of westernmost Michigan and along the north shore of Lake
Superior in Minnesota and Ontario. Different formation names are given to these
rocks in different areas (fig. 39). The total volcanic sequence reflects a long
period of volcanic activity with a complex history. Erosional debris derived from the lower part of the volcanic sequence occurs in the sedimentary rocks
interbedded with the lava flows of the Portage Lake Volcanics. The
interpretation is that the lower part of the sequence was being eroded at the
margins of a basin while the later flows were still being erupted into the
central part of the basin.

LAKE SUPERIOR REGION - distribution of selected rock units. (Fig. 39)

WHAT HAPPENED WHEN?

The geologic story of Isle Royale as presented up to this point has been largely
a description of the geology as we see it now. But how did it get this way? And
when? The search for answers to these questions involves considerable
interpretation of geologic observations made on Isle Royale and in the Lake
Superior region, together with numerous deductions based upon our accumulated geologic knowledge. Some
parts of the geologic history can be deciphered in considerable detail, and
other parts only incompletely because the geologic data are very spotty. The
first step toward the answers is to develop a time framework within which to
reconstruct events in the geologic past.

TIME AS A GEOLOGIC CONCEPT

A relative time scale, useful throughout the world, has been developed by
dividing the last approximately 570 million years of geologic time into periods.
The periods, which began with the Cambrian and end with the Quaternary, are
recognized largely from the fossil record (fig. 40). Rocks of the Cambrian
Period contain the earliest evidence of complex forms of life, which slowly
evolved through the subsequent periods into the life of the modern world. The
presence of distinctive fossils in various periods permits the correlation of rocks of similar
age. However, the near absence of fossils in Precambrian rocks - those older
than 570 million years - severely limits the use of fossils for the relative
dating of the Precambrian rocks, rocks which cover more than 80 percent of
geologic time. The rocks on Isle Royale are unfossiliferous and of Precambrian
age, and it is with this poorly classified segment of geologic time that we are
mostly concerned.

Fortunately, a means for measuring geologic time without fossils has been
developed from the long-known natural process of radioactive decay of certain
elements. From such radiometric age determinations, many rocks can now be
assigned ages in years, which for Precambrian rocks, in the near absence of
fossils, are virtually the only means for long-range correlations.

The uppermost, or youngest, Precambrian rocks in the Lake Superior region
consist of a thick sequence of volcanic and sedimentary rocks. This sequence has
been named the Keweenawan Supergroup because many of the formations that make it up were first
described on the Keweenaw Peninsula and adjacent parts of Michigan and
Wisconsin. The period of time during which the Keweenawan rocks were deposited
can be referred to informally as Keweenawan time, recognizing that the term has
usefulness only in the Lake Superior region where the Keweenawan rocks
themselves occur. The Keweenawan Supergroup has been informally divided into
lower, middle, and upper parts; formations exposed on Isle Royale, the Portage
Lake Volcanics and the Copper Harbor Conglomerate, are assigned to the middle
Keweenawan (fig. 41). We do not have sufficient data to determine the bounds of
Keweenawan time, but radiometric age determinations indicate ages in the general
range of 1,120-1,140 million years ago for the Keweenawan volcanic rocks. It is
from this point in the enormously distant past that we begin to unravel the
geologic history of Isle Royale.

KEWEENAWAN SUPERGROUP - classification in Michigan. (Fig. 41)

WHAT WAS THERE BEFORE
--- THE PRE-KEWEENAWAN

Rocks of Keweenawan age, as old as they are, rest upon still
older Precambrian rocks that are strikingly different. The older, or basement,
rocks are of many varieties, ranging from granite and other intrusive igneous
rocks to volcanic and sedimentary rocks, and cover a wide range in age. They
usually are both more highly metamorphosed, or altered from their original
state, and more strongly deformed than rocks of the Keweenawan sequence.
Basement rocks are not exposed on Isle Royale, but they are sufficiently well
exposed in areas both north and south of Lake Superior for their
nature to be studied and their relationship to the overlying Keweenawan sequence
to be determined.

The history of the pre-Keweenawan rocks is extremely complex, as might be
expected from the wide range in rocks and age; however, prior to the time the
Keweenawan rocks were deposited, much of what is now the Lake Superior region
had been reduced to an area of fairly low relief, and shallow seas covered large
parts of it. A thick sequence of marine sedimentary and volcanic rocks was then formed in those seas. In
addition to impure sandstone and shale, chemically precipitated sediments were
deposited, including dolomite and the iron-rich rocks that were later to make
the Lake Superior district world famous as a producer of iron ore. The material
forming the pre-Keweenawan volcanic rocks was erupted under water, where quick
quenching of the lava caused it to congeal in irregular-shaped globular or
ellipsoidal masses that accumulated one upon the other at the bottom of the sea
(fig. 42). These volcanic rocks are thus quite different from the uniformly
layered widespread sequence of Keweenawan flood basalts.

ELLIPSOIDAL STRUCTURES indicative of extrusion of lava under water, found in
Precambrian volcanic rock from northern Michigan. (Fig. 42)

Toward the close of pre-Keweenawan time, the rocks present were folded,
deformed, and eroded, quite drastically in some areas and only slightly in
others, and so the material forming the overlying Keweenawan volcanic and
sedimentary rocks was
laid down on the edges of the older beds in some localities.

THE ROCKS OF ISLE ROYALE --- THE KEWEENAWAN

The lowermost Keweenawan strata, which are not present on Isle Royale, consist
of a relatively thin sequence of sedimentary rocks, chiefly conglomerate and
sandstone, that lie on the preKeweenawan rocks. They have been interpreted as
shallow-water, near-shore deposits and are exposed in only a few localities
along the perimeter of the area of Keweenawan outcrops around the margins of the
Lake Superior basin. The lowermost few of the Keweenawan lava flows above these
sedimentary rocks contain ellipsoidal structures typical of those formed when
lava is erupted into a body of water. Such structures are found only near the
base of the lava sequence, however, and we conclude that the first few of the
flood basalt flows were erupted into a shallow body of water, which was soon
filled or disappeared for one reason or another, and that all later flows were
on virtually dry land. Furthermore, as mentioned earlier, the sedimentary rocks
interbedded with the lava flows show characteristics typical of those formed in
streams rather than in lakes or oceans.

The source of the lavas appears to have been along the axis of an arc-shaped
trough or system of fissures generally following the center of present-day Lake
Superior. The system was actually much more extensive, however, extending
southwest at least as far as Kansas and southeast into the southern peninsula of
Michigan, a total distance of over 1,000 miles. Great volumes of lava were
erupted from fissures near the axis of the trough and spread laterally and rapidly toward
both margins of the trough, finally ponding and cooling in place to form vast
sheets covering thousands of square miles. The Portage Lake Volcanics, as
exposed on the Keweenaw Peninsula and Isle Royale, represent roughly the upper
half of this volcanic pile (fig. 41).

The material forming the sedimentary rocks that are interbedded with the lava
flows of the Portage Lake Volcanics and that are in the Copper Harbor
Conglomerate and other Keweenawan formations above the lava sequence was
transported by streams into the trough or basin from highlands around its
margins. This evidence for inward flow of streams, contrasted with evidence
that the lavas flowed toward the margins of the basin, shows that there were, at
times, reversals of the prevailing slope over large areas and leads to the
concept of a basin subsiding as it was being filled (fig. 43). The surfaces of
the lava flows were horizontal or sloped gently toward the margins of the basin
as long as filling by lava kept pace with downwarping. When extrusion of the
lava was interrupted, however, continued downwarping produced inward slopes that
permitted sedimentary debris to be swept into the basin. Finally, with the
gradual demise of volcanic activity, continued subsidence permitted the
accumulation of the Copper Harbor Conglomerate and younger Keweenawan deposits
to form a thick sedimentary sequence above the volcanic rocks in the basin.

Sandstone of latest Keweenawan or earliest Cambrian age is exposed along much of
the shore of the southeastern part of Lake Superior and extends westward to the
Keweenaw Peninsula. The basin may have been largely filled by
this sandstone, together with similar sandstone exposed in the southwestern part
of the basin (fig. 39).

The gross synclinal form of the Keweenawan basin resulted from subsidence
coincident with filling of the basin rather than later folding by squeezing.
However, Keweenawan strata near the margins of the basin, as on the Keweenaw
Peninsula and Isle Royale, were subsequently steepened by upward movement on
major faults, the Keweenaw fault and the Isle Royale fault, thus accentuating
the synclinal structure (fig. 38).

SOME MINERALS OF SPECIAL INTEREST

Many interesting minerals occur on Isle Royale in addition to the basic rock-forming minerals that make up the bulk of the lava flows and other rocks on
the island. Among these are the native copper that played such an important part
in the early explorations of the island and chlorastrolite, the official state
gem of Michigan.

"These minerals as a group are known as secondary minerals, ones introduced
into the rocks after the rocks themselves were formed. They chiefly fill holes
in the volcanic rocks and the conglomerates or form veins filling fractures.
Some of these minerals, especially those occurring as amygdules in the volcanic
rocks, are quite attractive when polished and long have been sought by
collectors, although collection has been restricted since Isle Royale became a
national park. Fortunately for the collector, these minerals, except for
chlorastrolite, are equally or more abundant on the Keweenaw Peninsula or
elsewhere in the Lake Superior region.

Considerable evidence obtained during mining activities on the Keweenaw
Peninsula indicate that the copper and most other secondary minerals were
deposited from solutions that percolated upward through the rocks. We can only
speculate upon the ultimate source of the copper and other elements in the
mineralizing solutions, but one of the more generally accepted theories is that
those elements were "sweated" out of the lower part of the pile of
volcanic rocks after those rocks were warped downward in the Lake Superior
syncline, into a region of higher temperature and pressure. The elements then
migrated upward and were deposited as minerals in open spaces higher in the rock
sequence. If the source of the mineralizing solutions was indeed the deeper part
of the volcanic pile, then the source also would have been closer to the
Keweenaw Peninsula, for the axis, or deepest part of the Lake Superior syncline,
is closer to the Keweenaw Peninsula than to Isle Royale. This asymmetry of the
syncline may have been one of the more important factors in producing
economically valuable deposits of copper on the peninsula but not on Isle Royale. Among the
many secondary minerals on Isle Royale, those occurring most often are barite,
calcite, chlorite, copper, datolite, epidote, laumontite, natrolite, prehnite,
chlorastrolite (pumpellyite), quartz (including agate), and thomsonite. Only a
few of special interest on Isle Royale are described further in this report.
Information on the others can be obtained from many sources, including the
booklet "Rocks and Minerals of Michigan" (Poindexter and others,
1965).

NATIVE COPPER -
WIDESPREAD BUT NOT ABUNDANT Copper, like most other metals, most
commonly occurs in nature bound up with other elements into minerals such as
chalcocite(Cu2S), chalcopyrite (CuFeS2), and cuprite
(Cu2O), and such minerals
form the bulk of copper ores. Of all the metals that do occur in the native or
metallic state, copper is by far the most common; however, major concentrations
of native copper are rare, and so it is noteworthy that the most important
deposits known in the world are those of the Keweenaw Peninsula.

Economically valuable concentrations or deposits of native copper on the
peninsula can be divided into two broad groups - lode deposits and fissure
deposits. The lode deposits comprise mineralized conglomerate beds and the
vesicular tops of lava flows; in each case the primary porosity
of the rock or presence and continuity of open spaces was a factor in
determining where the ore would be deposited. The fissure deposits are along
fracture zones that generally cut across the beds. Some of the fissure deposits
contained great masses of metallic copper as much as hundreds of tons in weight.
Extensive Indian mining pits on the fissures later led prospectors to most of
the known deposits of this type. However, such deposits, rich as they were, have
been much less important than the lower grade but vastly larger lode deposits,
which have produced about 98 percent of the total copper mined in the Native
Copper district, about 5,400,000 tons.

COPPER NUGGET weighing 5,720 pounds found at a depth of 16 1/2 feet in a pit dug
by prehistoric Indians at the site of the Minong mine. Note the uneven surface
resulting from attempts to remove sections for implements (Burton collection,
Detroit Public Library). (Fig. 69)

Native copper is widely distributed on Isle Royale, but mineralization was
apparently too weak to develop large lode deposits. Except for the Island mine,
which was on a conglomerate lode, most of the prospects and short-lived mines on
the island were opened on small fissure deposits. In the fissure occurrences,
such as at the Minong, Siskiwit, and most other mines and prospects, native
copper occurs in nodules and irregular masses in highly altered rock in fracture
zones a few inches to many feet wide. Several large masses more than a ton in
weight were found at the Minong mine (fig. 69); these were rare and most pieces
mined were probably similar to that in figure 70, or smaller. Although
approximately 250 tons of copper were produced from the Minong mine, the copper
was too sparse and widely distributed to be mined profitably.

At the Island mine, near the west end of Siskiwit Bay, native copper occurs in
the matrix of a conglomerate, the only known occurrence of this type on Isle
Royale. The specimen illustrated (fig. 71), however, is much richer in copper
than average for the mine, and this mine too was a financial failure.

NATIVE COPPER MASS from the Minong mine. This is perhaps typical of specimens
from the fissure deposits on Isle Royale, but larger than average. (Fig. 70)

In other places in the park, most notably on the chain of islands south of Rock
Harbor, veins of quartz, prehnite, and calcite contain scattered grains of
copper. Native copper also occurs in amygdules in lava flows, especially in
prehnite amygdules.

CONGLOMERATE WITH NATIVE COPPER from the Island mine. The specimen has been
sawed and slightly polished so that the copper reflects light and shows up as
light-colored irregular-shaped patches. Specimen is 8 centimetres wide. (Fig. 71)

Rakestraw (1966) has described the historic mining ventures on Isle Royale and
what the visitor can see at some of the abandoned mine sites.

CHLORASTROLITE ---- MICHIGAN'S STATE GEM

Isle Royale has long been famous as the home of chlorastrolite, known informally
in rock-collecting and lapidary circles as "Isle Royale greenstone."
This usage of "greenstone" should not be confused with the use of the
same term for a volcanic rock with a greenish hue, such as makes up Greenstone
Ridge on the island. In 1972 the governor of Michigan signed a bill designating
chlorastrolite as the official state gem.

Chlorastrolite, meaning "green star stone," occurs as amygdules or
cavity fillings in certain of the lava flows on Isle Royale. When weathered out
of the lava flows, it can be found on some of the island beaches as pea-sized
pebbles, generally greenish in color. When polished, either by wave action on
the beaches or artificially, the "greenstones" generally exhibit a
distinctive and attractive mosaic or segmented pattern, sometimes referred to
as "turtleback" (fig. 72). The polished stones also commonly are
chatoyant - the property of having a luster resembling the changing luster of the
eye of a cat. Chatoyancy is probably best known in the gemstone called tiger eye
and is a property of translucent material that contains fibrous structures
capable of' scattering light. The grouping together of bundles of such fibers
produces the mosaic pattern of the "greenstones."

Chlorastrolite was first discovered on Isle Royale and named and described by C.
T. Jackson and J. D. Whitney in 1847. Long afterward it was found to be the same
material as another mineral, pumpellyite, first described from the Keweenaw
Peninsula in 1925 and named for Raphael Pumpelly, a pioneer student of the
minerals of the Keweenawan copper deposits. The material from the peninsula was
described in much greater detail than that from the island, and the name "pumpellyite" became deeply entrenched in the world mineralogical literature
long before it was realized that the material from both areas was
mineralogically the same. Consequently, pumpellyite has been adopted as the only
valid name for the mineral species, although chlorastrolite is still useful as a
term to designate the variety with the peculiar crystal habit of the Isle Royale
"greenstone." Pumpellyite is common in many parts of the world, but
the chlorastrolite variety is apparently rare outside of Isle Royale.

PREHNITE - THE LITTLE PINK PEBBLES

Prehnite is an abundant secondary mineral in some lava flows on Isle Royale and
elsewhere in the Lake Superior region. It occurs as amygdule fillings,
crosscutting veins, and as a replacement of earlier minerals or rock. Most of
the prehnite has a characteristic pale-green to white color, but that in
amygdules is commonly light to dark pink or variously mottled in pink and green.
The pink prehnite superficially resembles thomsonite, with which it has commonly
been confused: most of the so-called thomsonite from Isle Royale is actually
pink prehnite, including material from Thomsonite Beach on the north side of the
island. The prehnite does not develop the spectacular patterns and color
variations present in gem-quality thomsonite, which explains why "Isle
Royale thomsonite" has always been considered to be of inferior quality.
Nevertheless, the prehnite amygdules polish nicely and are attractive in
themselves.

The pink prehnite amygdules, which most commonly range in size from 1/2 to 1 centimeter, are more resistant to erosion than the volcanic rock matrix within
which they have formed. As a result, beach pebbles containing them often have a knobby appearance, the
amygdules projecting above the general surface of the matrix (fig. 73). Where
the prehnite amygdules weather completely free from the matrix, they may make up
a fair percentage of the fine gravel on beaches near the prehnite-bearing
outcrops.

The amygdules in figure 74 are typical and illustrate the radiating fibrous habit, with the occasional development
of "eyes," that has probably been a factor in their confusion with
thomsonite. The pink color of the prehnite is due to internal reflections from
finely disseminated native copper inclusions, and the color intensity is related
to the distribution, abundance, and grain size of the inclusions.

AGATE - AN ARRAY OF COLORS

Perhaps the best known gemstones of the Lake Superior region are agates.
Although they are most plentiful near where they weather from the enclosing
rocks, glacial transport has caused them to be widely distributed, and there are
few pebble beaches where one cannot be found.

Agate is a subvariety of chalcedony (fibrous quartz) with a distinct banding in
which successive layers differ in color and in degree of translucency. It most
commonly originates as a cavity filling in volcanic rocks either as amygdules in
volcanic flows or as irregular-shaped masses in volcanic tuffs; both types occur
in the Lake Superior region, including Isle Royale.

One of the most striking examples of an agate-bearing lava flow in the park is
the Amygdaloid Island Flow. This flow, exposed only on Amygdaloid Island, has
rather abundant rounded or almond-shaped agates with a characteristic flesh-pink
color and commonly massive quartz or vuggy centers (fig. 75). Agates from an
individual flow tend to be somewhat similar in appearance; for example, those
from the Amygdaloid Island Flow are pinkish, whereas those from the Long Island
Flow tend to be bluish. In general, agates from the rocks on Isle Royale are paler and have less color contrast between color bands than those found in some
other parts of the Lake Superior region.

The volcanic tuff that overlies the Greenstone Flow also contains numerous
agates with a pink or red cast (fig. 76). Unlike the ovoid agates typically
occurring in the lava flows, these agates tend to have very irregular shapes
similar to the so-called thunder-egg agates occurring in welded tuffs of the
Columbia River Plateau in the northwestern United States, and they probably had
a similar origin. The welded tuffs formed from volcanic ash that fell in a hot
plastic condition that permitted it to be fused or welded into a generally
cohesive mass. The agates themselves are interpreted as chalcedony deposited in
cavities formed by pockets of gases accumulating during cooling, shrinkage, and
partial crystallization of the tuff.

Because of their hardness, agates are more resistant to erosion than the
enclosing rock and tend to become concentrated in gravel and other surficial
deposits both ancient and modern. Thus agates are present in the conglomerates
interbedded with the lava flows on the island and in the Copper Harbor
Conglomerate, as well as in the modern surficial deposits. Most of the agates on
beaches near conglomerate outcrops have, therefore, gone through two cycles of
weathering and release from their host rock - first from the volcanic rock in
which they originated and then from the conglomerate in which they were
subsequently incorporated. In addition to beach agates derived from nearby
bedrock, some have been winnowed from glacial till by wave action and may have
been transported a long distance from their original source; they commonly are
quite different in some way from the majority of the agates on a given beach.

AGATE typical of that in the volcanic tuff above the Greenstone Flow. (Fig. 76)

WHAT THE FUTURE HOLDS?

As we pick up an agate from the beach and admire its color and pattern, we can
reflect upon the enormous amount of change that has taken place since it
originated as an amygdule in a lava flow a billion or so years ago. The time from the volcanic eruptions to the
coming of the glacier that may have brought that agate to its resting place,
however, was itself enormous, even in the geologic sense - nearly one-fourth the
age of the earth itself. And although things are relatively quiet on Isle Royale
at the present time, geologically speaking, changes will continue to take place
slowly but certainly. Storm waves will continue to batter the shoreline cliffs,
wearing them back. Winter frost will loosen rock fragments on the hillsides for
the summer rains to wash downslope. The streams will deposit silt in the
landlocked lakes until they are filled and cease to exist as bodies of water.

These continuing changes take place so slowly that they are nearly imperceptible
to us. They are quite different from the cataclysmic eruption of a volcano or
the steady advance and envelopment of an ice sheet. But perhaps this is just a
quiet moment in the continuing geologic history of Isle Royale. While renewed
volcanic activity is unlikely, many scientists believe that we are now in an
interglacial epoch and that massive ice sheets will again form on the Canadian
Shield to move inevitably southward, again modifying the earth's surface as they
grind their way over it. Perhaps the future glacier might even bring some new
agates to the beach for another visitor to admire.